...I could agree with you more!
There is a slight difference when I say it, but, to be frank, there's only so much emphasis I can do with my dialect without the emphasis on the -'ve sounding ridiculous. When I say 'of', it much more clear.
Hahaha fuck, that's a great way to use it actually. Too bad people don't use it that way but damn, this was very impressively cool.
Well, the whole point of saying I couldn't care less is to convey the message that you don't. In my experience, it is often met with an expression of dismay, as though I've just committed the most horrible atrocity imaginable.